If we're being honest, the presidential race between Hillary Clinton and Donald Trump is effectively over. Which means that the big fight over the next 15 days is for control of the Senate, where Democrats need a net gain of four seats to retake control.

That prospect is looking more and more likely of late - thanks in large part to the Trump collapse at the top of the ticket, which appears to be dragging down the likes of Richard Burr in North Carolina, Kelly Ayotte in New Hampshire and Joe Heck in Nevada.

What few people talk about - but should - is that this could be a very short-lived majority for Senate Democrats, as the 2018 playing field is remarkably bad for them.

The raw numbers are stunning: 25 Democratic - or Democratic-affiliated independents - are up for re-election compared with just eight Republicans. That's as lopsided an election cycle as you'll ever see.

But a look inside the numbers makes the Democrats' challenge in 2018 all the more daunting. Fully 20 percent of the 25 Democratic seats are in states that Mitt Romney carried in 2012 (and even Trump is likely to carry on Nov. 8): Indiana, Missouri, Montana, North Dakota and West Virginia.

All five Democratic incumbents in those states are expected to run for re-election, a prospect that gives Democrats a chance in each. But with 2018 looking almost certain to be the first midterm election of a Hillary Clinton presidency, it's hard to see how her party avoids major losses in red states.

Some important historical context: In the first midterm election of his term, in 2010, President Barack Obama lost 63 House seats and six Senate seats. Bill Clinton lost 52 House seats and eight Senate seats in his first midterm election as president in 1994. George W. Bush was the exception, with his party gaining eight House seats and two Senate seats in 2002, although that election was heavily influenced by the Sept. 11, 2001, terrorist attacks.

While those five states are deeply endangered for Democrats, they are far from the only possible Republican targets. Sen. Bill Nelson, D-Fla., has said he plans to run for re-election, but retirement rumors still swirl. Even if he stays put, he could face a very competitive race. Sen. Sherrod Brown, D-Ohio, cruised to a victory in 2012, but the Buckeye State's natural competitiveness suggests he will face a serious Republican opponent. Wisconsin, too, where Tammy Baldwin is in her first term, could see a real race.

Then there is the wild card of Virginia. Tim Kaine's likely ascension to the vice presidency will force Gov. Terry McAuliffe (D) to appoint a replacement. That person will serve until November 2017, when he or she - Rep. Bobby Scott is the leading name - will face a special election for the remaining year on Kaine's term. Whoever wins that special election will have to stand for a full six-year term in November 2018.

While Democratic problems are everywhere on the map, Democratic opportunities are few and far between. Sen. Dean Heller (Nev.) is an obvious target given the state's competitiveness at the federal level. But after that, the pickings get very, very slim. Sen. Jeff Flake, R-Ariz., might be a Democratic opportunity, given the state's increasing competitiveness in 2016. But Flake has been one of the most outspoken anti-Trump voices within his own party and won't be easily caricatured as a Trump Republican.

In short: Even if Democrats win the Senate in 2016, it could be a blink before they are back in the minority. Which means that President Hillary Clinton will have two years to work with a friendly Senate before things get much, much tougher for her in Congress.